Micro-electro-mechanical system devices (MEMS) are usually made from inorganic materials using semiconductor technologies. Examples of commonly used inorganic material for MEMS devices include silicon, silicon oxide, silicon nitride, and aluminum. These inorganic materials possess high surface energy. As a result, the surfaces often stick together when they come into contact. This problem is particularly significant because the surface-area to volume ratio scales with the inverse of device dimension and this ratio is very large for MEMS devices with typical dimensions on the micrometer scale.
A well-known surface related problem in the fabrication and operation of MEMS devices is stiction, which occurs when the surface adhesion force overcomes the mechanical restoring force of microstructures. Stiction is one of the leading causes of device failure in the MEMS industry. One example of a MEMS device currently in commercial use is the digital mirror device (DMD) of Texas Instruments. The DMD consists of ˜1,000,000 micro-mirrors. During operation, each mirror is rotated ±10° to reflect light from a source to the screen; this rotation brings the mirror assembly (specifically, the tip of the yoke on which the mirror is mounted) into contact with the substrate and stiction can occur. Another example of a commercial product utilizing MEMS technology is the airbag sensor of Analog Devices. The airbag sensor, also called accelerometer, consists of a movable component which responses to changes in inertial during collision. However, the movable component may become stuck to other fixed component in its immediate environment resulting in device failure.
One approach to solve the stiction problem has been to apply a passivation, organic coating to the surfaces of MEMS devices. Organic coatings consisting of hydrocarbon or fluorocarbons are generally characterized by low surface energy. When surfaces with such low energy coatings come into contact, the adhesion energy is substantially reduced as compared to the uncoated, inorganic surfaces. The lowering of surface energy helps to alleviate the stiction problem.
Another approach to apply a passivating coating onto a MEMS devices involves the introduction of an organic material, phenyl-siloxane in particular. Still another approach has been to utilize a combination of an organic material and moisture with a MEMS device in a sealed package and heating the package to a high temperature to form a passivation coating on the surface of the MEMS device. This type of coating has been applied to accelerometers (airbag sensors) where contacts between different components on a MEMS device are infrequent. Thus emphasis of this method has been on thermal stability of the coating to be compatible with packaging temperature, rather than achieving the lowest possible surface energy or the highest mechanic stability.
Therefore, there remains a need in the art for improved methods and coatings applicable to a MEMS device that provide for robust coatings and the reduction or, preferably, elimination of stiction.